This application claims priority of Korea patent Application No. 2000-65483, filed on Nov. 6, 2000.
(a) Field of the Invention
The present invention relates to a method for controlling a continuously variable transmission (CVT), and more particularly to a method capable of automatically controlling a speed ratio between economic and power modes without manual mode selection.
(b) Description of the Related Art
An automotive transmission is a device for transmitting engine torque to a drive shaft of a vehicle in variable speed ratios, and transmissions are usually classified into manual, automatic, and continuously variable transmissions (CVT).
Unlike manual and automatic transmissions that adapt a step variable is gear mechanism to provide fixed speed ratios, the CVT can vary the speed ratio continuously.
Accordingly, engine revolution speed can be optimally selected such that an engine operates at a preferred revolution-per-minute (rpm) relative to one of intended conditions such as maximum mileage, maximum output, minimum noise, minimum toxic gas emission, etc.
Typically, the speed ratio of the CVT is determined by engine torque-rpm maps that are preset according to intended conditions such as maximum mileage (economy mode) and maximum power (power mode).
FIG. 4 is a graph showing engine performance curves. Curve 1 indicates a plurality of iso-Brake Specific Fuel Consumption (iso-BSFC) curves and P0 indicates a point where the fuel consumption is the lowest. Curve 2 indicates a plurality of iso-power curves, which are equivalent to graphs of the equation y=power/x with various values of power, because the engine power is calculated by multiplying the engine rpm by the engine torque. Curve 3 indicates a maximum mileage curve, which passes through the lowest fuel consumption point P0. Curve 4 indicates a maximum power control curve.
In the CVT, the speed ratio can be randomly shifted in an available range such that the engine rpm and the engine torque can be randomly set regardless of the vehicle speed.
As a result, the CVT is controlled in such a way that the engine performs with a specific torque and rpm along the maximum mileage curve C3 to provide maximum mileage of a vehicle, and it performs with another specific torque and rpm along the maximum power control curve C4 for maximum power driving.
Accordingly, the speed ratio of the CVT is determined on the basis of a throttle valve opening and vehicle speed values specified in the map corresponding to the control mode that are determined relative to the intended driving condition, such as maximum mileage or maximum power.
Therefore, in order to accommodate the driver""s intention, a conventional CVT control method requires mode selection input from the driver, and the mode selection process can cause the speed ratio to be abruptly changed, resulting in shift shock, vibration, and engine shaking.
The present invention has been made in an effort to solve the above problems of the prior art.
It is an object of the present invention to provide a CVT control method capable of determining an optimal speed ratio that lies betweenspeed ratios of the economy and power modes without manual mode selection.
To achieve the above object, a CVT control method of the present invention comprises the steps of calculating a target speed ratio of a CVT between maximum and minimum values of a plurality of speed ratios preset corresponding to a plurality of engine operation modes and controlling the CVT according to the target speed ratio.
The step of calculating the target speed ratio includes the steps of calculating a driving pattern index on the basis of throttle opening change rate, throttle operation frequency, and vehicle acceleration, and calculating the target speed ratio on the basis of the driving pattern index.
The target speed ratio can be calculated using a formula such as Tm=(Tpxe2x88x92Te)X+Te, where X is the driving pattern index, Te is an economy mode speed ratio, Tp is a power mode speed ratio, and Tm is the target speed ratio between Te and Tp.
The driving pattern index X can be calculated using a formula:   X  =            (                        α          xc3x97                      a            A0                          +                  β          xc3x97                      b            B0                          +                  γ          xc3x97                      c            C0                              )        /    3  
where, xcex1, xcex2, and xcex3 are proportional weights;
a, b, and c are respectively a learned throttle opening change rate, learned throttle operation frequency, and learned vehicle acceleration; and
A0, B0, and C0 are respectively a preset throttle opening change rate, preset throttle operation frequency, and preset vehicle acceleration.
In addition, the CVT control method according to the present invention can further includes the steps of determining if a learning condition is satisfied, and learning the driving pattern index when the learning condition is satisfied.
The learning condition can be defined as ""The present change rate of throttle opening is different from an average change rate of throttle opening up to this point by more than a predetermined difference, or the present throttle operation frequency is different from an average throttle operation frequency up to this point by more than a predetermined frequency, or the present acceleration is different from an average acceleration up to this point by more than a predetermined acceleration.
Furthermore, the CVT control method of the present invention can further include the step of determining whether the vehicle is running at a constant speed, wherein the step of calculating a driving pattern index is performed if the vehicle is not running at a constant speed.
When the vehicle is running at a constant speed, the target speed ratio is determined to be one of default engine operation modes, which is preferably an economy mode.